This invention relates to an ink jet system having a slit-like ink jet port and, more particularly, to an ink jet printer of an electrostatic acceleration type.
Heretofore, there has been a well-known ink jet printer, which forms record dots on a recording medium by causing liquid ink to fly thereto. This ink jet produces less noise during the recording operation. In addition, since ink is caused to be attached directly to the recording sheet, neither developing nor fixing processes are necessary. However, the ink jet port is liable to be clogged due to drying and solidification of the liquid ink, leading to instable recording operation. Various techniques, therefore, have been studied to solve the above problem inherent in the ink jet printer as noted above. An ink jet printer having a sit-like ink jet port has been developed. In this case, the clogging of the ink jet port with ink is less likely to happen because the port is slit-like. For these reasons, this type of ink jet printer has been attracting interest.
In an electrostatic acceleration type ink jet printer using the slit-like ink jet port as noted above, a plurality of recording electrodes are provided on the inner surface of the slit-like ink jet port, which has a gap dimension of approximately 100 .mu.m and a length of approximately 200 mm, at a density of approximately 8 electrodes per millimeter. When a high pulse voltage is applied to selected recording electrodes corresponding to printing points, a high electric field is produced between each selected recording electrode and a back electrode facing the ink jet port. As a result, ink in the vicinity of each selected recording electrode under the high applied voltage is caused by electrostatic forces to jet toward the back electrodes. In this way, dots of ink are produced on a recording sheet provided in front of the back electrode in correspondence to the recording signal.
As the method of applying a high pulse voltage to a plurality of selected recording electrodes, there is one, in which individual recording electrodes are connected to respective high voltage pulse generators and these high voltage pulse generators are driven selectively according to recording data. This method has a problem in that the recording electrodes and high voltage pulse generators have to be connected to one another by a large number of leads.
Japanese Patent Laid-Open Publication No. 60-250,962 discloses an improved printer system. In this system, individual recording electrodes are connected through respective photoconductive sections to a first common ink jet control electrode and are also connected through respective fixed resistors to a second common ink jet control electrode. A D.C. high voltage is applied between the first and second common flying control electrodes, and as an optical signal corresponding to recording data illuminates corresponding photoconductive sections the potential on each recording electrode is changed according to the recording data.
This system makes use of the fact that the resistance of each photoconductive section is varied according to the intensity of light incident on the photoconductive section to vary the voltage division ratio of a voltage divider constituted by the resistance between the first and second ink jet control electrodes, thus giving rise to a difference in potential between the recording electrodes connected to photoconductive sections which are illuminated by light and those which are not illuminated. More specifically, assuming that the electric resistance between the recording electrode and the first ink jet control electrode is changed from Rd to Rp with illumination of associated photoconductive section with a light signal, potential Vp on a recording electrode with the associated photoconductive section illuminated by light and potential Vd on a recording electrode not illuminated by light are given respectively as ##EQU1## where Rc is the constant electric resistance between the recording electrode and the second ink jet control electrode, +V.sub.1 is the voltage applied to the first ink flying control electrode, and -V.sub.1 is the voltage applied to the second ink flying control electrode.
Change .DELTA.V in the recording electrode potential before and after the illumination of the photoconductive section with the light signal is thus expressed as ##EQU2##
Thus, the jetting of ink is controlled according to the optical signal by adjusting V.sub.1, Rc, Rd and Rd such that .DELTA.V has a predetermined value.
The recording system which permits control of the jetting of ink with the illumination of the photoconductive sections with an optical signal as noted above, has a possibility that it permits provision of a new copier, which does not require complicated mechanisms for developing, fixing and other steps as are necessary in the ordinary electrophotographic duplicator or the like, and the provision of a practical system has been desired.
In the ink jet recording system as noted above, as is obvious from equation 3, the change .DELTA.V in the recording electrode potential is increased in proportion to the voltage 2V.sub.1 applied between the first and second high voltage application electrodes. The potential difference between the recording electrode and the back electrode thus can be increased at the time of the jetting of ink, thus increasing the electrostatic force given to ink. However, the photoconductive sections and fixed resistors provided between the recording electrodes and high voltage application electrodes have breakdown voltage limitations, so that a limitation is imposed on the recording electrode potential change .DELTA.V. Therefore, it is necessary to make the distance between the ink jet port and back electrode to be sufficiently small. However, if the distance between the ink jet port and back electrode is too small, there arises a problem that ink is attached continuously to the recording sheet. Even where the distance between the ink jet port and back electrode is appropriately set, if a physical property of ink such as the surface tension thereof is changed, the properties of the ink jet printer are changed to give rise to the problem noted above.
The ink jet recording system noted above has a further problem in the case when ink is jetted simultaneously from adjacent ones of a plurality of recording electrodes provided on the inner surface of the slit-like ink jet port. In such a case, adjacent ink jet toward the recording sheet exert electrostatic forces of repulsion to one another. This results in a disturbance of the direction of the ink jet, thus spoiling the quality of the produced copy image. This problem is also posed in the system where a signal is applied to recording electrodes by selectively driving high voltage pulse generators. In a system disclosed in Japanese Patent Laid-Open Publication No. 56-167,476, the above problem is solved by providing a shift of high recording pulse voltage application timing between most adjacent recording electrodes.
In a recording system, in which the jetting of ink is controlled by leading light reflected from an original directly to photoconductive sections, however, it is thought to be impossible to provide a shift in the high pulse voltage application timing between adjacent recording electrodes. More specifically, in an ink jet system, which has a slit-like ink jet port and is provided with a circuitry including photoconductive sections, there is a problem that ink jets from adjacent recording electrodes exert electrostatic forces of repulsion on one another and disturb the direction of the ink jet, thus spoiling the quality of the record image.
Meanwhile, ink jets are accelerated from the slit-like ink jet port toward the back electrode by the potential difference between the recording electrode and the back electrode. This potential difference is influenced by various factors such as the distance between the ink jet port and back electrode, applied pulse voltage and surface tension of the ink drop, but, roughly, it is required to be as high as 1 to 3 kV. Therefore, the prior art ink jet recorder of this type is prone to an occurrence of spark discharge between the recording electrode and the back electrode, and the ends of the recording electrodes are liable to be broken by spark discharge. The phenomenon of spark discharge will be described in further detail. Ink in the vicinity of the slit-like ink jet port experiences a pulling force, which is produced by the high pulse voltage application and tends to pull ink toward the back electrode, and a surface tension, which tends to pull ink back toward the ink jet port. These two different forces are exerted and in opposite directions. Consequently, a wave is produced on the surface of the ink in the ink jet port, and there are instances in which the ends of the recording electrodes are not covered by ink. This promotes the occurrence of spark discharge. When a spark discharge occurs, the ends of the recording electrodes are broken, resulting in an increase of the distance between recording electrode and back electrode beyond a predetermined value. Consequently, failure of flying of ink results, giving rise to deterioration of image quality with time.